This invention relates to composite fiber assemblies. More particularly, this invention relates to composite nanofiber assemblies that can be produced from a polymeric matrix material.
Various techniques are known in the textile field for the creation of fibers. Melt-blowing, the nanofibers by gas jet (NGJ) technique, and electrospinning are included among these techniques. In a melt-blowing process, a stream of molten polymer or other fiber-forming material is typically extruded into a jet of gas to form fibers. The resulting fibers are typically greater than 1,000 nanometers in diameter, and more typically, greater than 10,000 nanometers in diameter.
A technique and apparatus for forming fibers having a diameter of less than 3,000 nanometers according to the NGJ technique is described in U.S. Pat. Nos. 6,382,526 and 6,520,425, the disclosures of which are hereby incorporated by reference.
The electrospinning of liquids and/or solutions capable of forming fibers, also known within the fiber forming industry as electrostatic spinning, is well known and has been described in a number of patents as well as in the general literature. The process of electrospinning generally involves the creation of an electrical field at the surface of a liquid. The resulting electrical forces create a jet of liquid that carries electrical charge. Thus, the liquid jets may be attracted to other electrically charged objects at a suitable electrical potential. As the jet of liquid elongates and travels, it will harden and dry. The hardening and drying of the elongated jet of liquid may be caused by cooling of the liquid, i.e., where the liquid is normally a solid at room temperature; evaporation of a solvent, e.g., by dehydration, (physically induced hardening); or by a curing mechanism (chemically induced hardening). The produced fibers are collected on a suitably located, oppositely charged receiver and subsequently removed from it as needed, or directly applied to an oppositely charged or grounded generalized target area.
Fibers produced by this process have been used in a wide variety of applications, and are known, from U.S. Pat. No. 4,043,331 to be particularly useful in forming non-woven mats suitable for use in wound dressings. One of the major advantages of using electrospun fibers in wound dressings, is that very thin fibers can be produced having diameters, usually on the order of about 50 nanometers to about 25 microns, and more preferably, on the order of about 50 nanometers to about 5 microns. These fibers can be collected and formed into non-woven mats of any desired shape and thickness. It will be appreciated that, because of the very small diameter of the fibers, a mat with very small interstices and high surface area per unit mass, two characteristics that are important in determining the porosity of the mat, can be produced.
Medical dressings formed using non-woven mats of these polymeric fibers may provide particular benefits depending upon the type of polymer or polymers used, as taught by U.S. Pat. No. 4,043,331. A wettable, or hydrophilic, polymer, such as, for example, a polyurethane may be used, or a non-wetting, or at least weakly hydrophobic, polymer such as, for example, a saturated polyester, may be employed. Where the dressing is formed from a wettable polymer, blood or serum escaping from the wound tends to penetrate the dressing and the high surface area encourages clotting. Such dressings could be used as emergency dressings to halt bleeding. On the other hand, where the dressing is formed from a non-wetting polymer, and if the interstices between the fibers are sufficiently small, i.e., on the order of less than about 100 nanometers, tissue fluids, including blood, tend not to permeate the dressing. Consequently, the fluids are retained adjacent to the wound where clotting will occur. Subsequent removal of such a dressing is facilitated by the absence of blood clots permeating the dressing material. Still further, U.S. Pat. No. 4,043,331 suggests that such dressings have the advantage that they are usually sufficiently porous to allow interchange of oxygen and water vapor between the atmosphere and the surface of the wound.
Besides providing variability as to the diameter of the fibers or the shape, thickness, or porosity of any non-woven mat produced therefrom, the ability to electrospin the fibers also allows for controlled variations in the composition of the fibers, their density of deposition and their inherent strength. The above-identified U.S. patent indicates that it is also possible to post-treat the non-woven mats with other materials to modify their properties. For example, one could increase the strength of the mat using an appropriate binder or increase water resistance by post-treating the mat with silicone or other water-resistant material, such as perfluoro alkyl methacrylate. Alternatively, strength may be increased by utilizing fibers of polytetrafluoroethylene (PTFE).
By varying the composition of the fibers being formed, fibers having different physical or chemical properties may be obtained. This can be accomplished either by spinning a liquid containing a plurality of components, each of which may contribute a desired characteristic to the finished product, or by simultaneously spinning, from multiple liquid sources, fibers of different compositions that are then simultaneously deposited to form a mat. The resulting mat, of course, would consist of intimately intermingled fibers of different material. A further alternative noted in the above-referenced U.S. patent is to produce a mat having a plurality of layers of different fibers of different materials (or fibers of the same material but different characteristics, e.g. diameter), as by, for example, varying the type of fibers being deposited on the receiver over time. For example, wettable and non-wetting polymers each offer additional properties that may be desirable in different applications. Wettable polymers tend to be highly absorbant but provide mats that are relatively weak, while non-wetting polymers tend to be non-absorbent but provide mats that are relatively strong. In some applications, such as medical dressings, for example, it may be desirable to use a combination of wettable and non-wetting polymer layers in a single article. The wettable polymer layer or layers contribute a relatively high level of absorbency to the article while the non-wetting polymer layer or layers contribute a relatively high level of strength. Use of such a laminate-type structure, however, suffers from the disadvantage that the hydrophobic layer can form a barrier to liquids and interfere with the absorption of liquid by the wettable layer. Additionally, upon absorption of liquid, the wettable polymer layer will weaken and misalignment, slipping, or even separation of the layers may occur, resulting in failure of the integrity of the article.
U.S. Pat. No. 4,043,331 indicates that strong, non-woven mats comprising a plurality of fibers of organic, namely polymeric, material may be produced by electrostatically spinning the fibers from a liquid consisting of the material or its precursor. These fibers are collected on a suitably charged receiver. The mats or linings formed on the receiver can then be transferred and used alone or in conjunction with other previously constructed components such as, for example, mats of woven fibers and backing layers to provide a wound dressing of desired characteristics. For instance, in producing wound dressings, additional supports or reinforcement such as mats or linings of fibers, or backing layers may be required in order to adhere the wound dressing to the skin and to provide other desirable properties to the wound dressing. As an example, a mat or lining of woven fibers may contain materials having antiseptic or wound-healing properties. Surface treatments of the already formed non-woven mats may also provide added benefits in the production of such wound dressings. However, U.S. Pat. No. 4,043,331 does not provide a medical dressing that adheres to undamaged skin only. It also does not provide a single-component dressing that can adhere to a desired area of a patient, or a dressing comprised of composite fibers that vary in their composition along their length.
It has also been described in PCT International Publication No. WO98/03267 to electrostatically spin a wound dressing in place over a wound. In such a use, the body itself is grounded and acts as a collector of the electrospun fibers. This method of synthesizing a wound dressing allows for solution of some of the problems associated with bandage and gauze storage and preparation. It is well known for example, that gauze and bandages must be stored and maintained in a sterile environment in order to offer the greatest protection in healing wounds. If the gauze or bandages are not sterile, these products offer little help in protecting the wound. Electrospinning a wound dressing in place, over a wound, from a sterile liquid, eliminates these problems.
Electrospinning a wound dressing in place over a wound, however, limits the types of solvents that may be used to only those solvents that are compatible with the skin or other tissue to which the dressing is applied. Examples of such solvents include water, alcohols, and acetone. Likewise, because the types of usable solvents are limited, the types of additives, such as, for example, absorbents, bactericides, and antibiotics, that may be used in conjunction with the polymer are also limited to those that are soluble, or form a stable dispersion in the particular solvent used. Similarly, the types of polymers that may be used are also limited to those that are soluble in a skin- or tissue-compatible solvent. Biocompatible polymer/solvent combinations include, for example, poly(ethylenimine)/ethanol, poly(vinylpyrrolidone)/ethanol, polyethylene oxide/water, and poly(2-hydroxymethacrylate)/ethanol+acid. While fibers from such a combination are non-reactive in their state as spun, exposure of the fibers to fluids, either from a wound or from external sources, may cause a local pH change from a neutral or nearly neutral pH to one that is acidic or alkaline, depending on the composition of the fiber. For example, when poly(ethylenimine) fiber is exposed to fluid, it will participate in proton transfer, resulting in an alkaline pH in the fluid contacting the polymer. The creation of an undesirable pH environment may cause side effects, such as slow wound healing.
An electrospun fiber containing a substantially homogeneous mixture of a hydrophilic polymer, a polymer which is at least weakly hydrophobic, and optionally, a pH adjusting compound has been described in International Publication No. WO 01/27365, the disclosure of which is incorporated herein by reference. The fibers may be deposited directly on their intended usage area without first applying the fibers to a transient, charged receiver or subjecting it to other intermediate fabrication steps. The resulting fibers, however, do not provide a dressing which adheres to undamaged skin only.
Therefore, the need continues to exist for a medical dressing or other non-woven mat or membrane that is capable of adhering to a dry substrate but will not adhere to a wet surface such as a wound or to wet tissues that form in the early stages of wound healing. A need also exists for a medical dressing that can provide properties resulting from a variation in the composition of the individual fibers of the dressing over their length.